The networking of control units, sensors and actuators using a communications system or a bus system, that is to say, a communications link, has increased dramatically in recent years in modern motor vehicle manufacturing and in engineering, especially in the machine tool sector, that is to say, also in automation technology. By distributing functions among several control units it is possible in these cases to obtain synergetic effects. The term distributed systems is used for this. Communications between various stations is increasingly being carried out via a bus system, that is to say, a communications system. The communications traffic on the bus system, access and receiving mechanisms, and error handling are governed by a protocol. A known protocol for this is the CAN protocol or the TTCAN protocol or the FlexRay protocol, with the FlexRay protocol specification V2.0 currently being used as the basis. The FlexRay is a rapid, deterministic and error-tolerant bus system, especially for use in a motor vehicle. The FlexRay protocol operates according to the time-division-multiple-access (TDMA) method wherein the components, that is to say, users, and the messages to be transmitted are allocated fixed time slots in which they have exclusive access to the communications link. This is implemented in a comparable manner also in the case of TTCAN. The time slots are repeated in a fixed cycle, and therefore the time at which a message is transmitted over the bus may be exactly predicted and bus access takes place deterministically. To make optimum use of the bandwidth for message transmission over the bus system, FlexRay divides the cycle into a static part and a dynamic part. The fixed time slots are situated in the static part at the beginning of a bus cycle. In the dynamic part, the time slots are allocated dynamically. In the latter, exclusive bus access is made possible only for a short time (so-called minislots) in each case. Only if a bus access takes place within a minislot is the time slot lengthened by the required time. In that manner, therefore, bandwidth is used only when it is actually needed. FlexRay communicates via two physically separate lines each with a maximum data rate of 10 MB per second. The two channels correspond here to the physical layer, especially of the OSI (open system architecture) layer model. They are mainly used for redundant and therefore error-tolerant transmission of messages, but are also able to transmit different messages, which would then double the data rate. FlexRay may also be operated, however, with lower data rates.
In order to implement synchronous functions and to optimize bandwidth by using small intervals between two messages, the distributed components in the communications network, that is to say, the users, need a common time basis, the so-called global time. For clock synchronization, synchronization messages are transmitted in the static part of the cycle, with the local clock time of a component being corrected using a special algorithm conforming to the FlexRay specification, in such a way that all local clocks run synchronously with a global clock. That synchronization takes place in a comparable manner also in a TTCAN network.
A FlexRay node or FlexRay user or host contains a user processor, that is, the host processor, a FlexRay controller or communications controller and, in the case of bus monitoring, a bus guardian. The host processor, that is, the user processor, supplies and processes the data transmitted via the FlexRay communications controller. For communication in a FlexRay network, messages, or message objects, may be configured with, for example, up to 254 data bytes. In order to transmit those messages, or message objects, between the physical layer, that is, the communications link, and the host processor, a communications module, e.g., a communications controller, is used.
An object of the present invention is to provide a message memory for a communications module of a bus system, which message memory supports the transmission of messages in an optimum manner.